This invention relates to laser mirrors and more particularly to laser mirrors subject to thermal distortion and arranged and adapted to resist thermal distortion detrimental to laser action.
In the field of lasers, it has long been known that the alignment and optical figure of the mirrors are of critical importance in order to maintain maximum output. Frequently, laser mirrors after being correctly aligned and particularly mirrors used in chemical reaction and gas lasers wherein they are exposed to high energy laser beams as well as hot gases as described above, are subjected to heating which cause misalignment and distortion.
If one surface of a laser mirror is exposed to a uniform heat flux resulting, for example, from radiative and/or aerodynamic heating, conduction produces a temperature gradient in the material normal to its exposed surface. In the absence of external restraint, the differential thermal expansion results in spherical bending. Where a high quality optical system is required or desired, surface distortion should not be greater than a small fraction of the wavelength of the radiation of, for example, about 10% at the most. In certain gas laser systems, distortion should be kept at a value less than preferably one, or at most, several microns. Gas lasers such as, for example, gas dynamic and electrically pumped CO.sub.2 lasers are capable of extremely high output powers if mirror distortion does not limit obtainable power output.
Water cooled mirrors are commonly used to direct high energy laser beams. The laser beam incident on its water cooled front face causes a temperature rise of the front face which causes it to thermally expand. The back member of the mirror is made stiff to resist the bending caused by the expansion of the front face. However, the back is not infinitely stiff and, consequently, the mirror bends into a convex shape. For an incident laser beam whose energy absorbed by the mirror is azimuthally uniform, the shape of the bent mirror has no azimuthal distortion when the beam is normally incident to the mirror. A laser beam which is both azimuthally and radially uniform in absorbed intensity on the mirror will bend the mirror into a paraboloid, which requires only refocus correction.
However, if such a water cooled mirror is used for "folding" the laser beam, such that the incident beam direction is no longer normal to the surface, then the mirror will introduce astigmatism into the reflected beam. This can be exemplified by considering as an example a circular beam of uniform intensity, which is to be redirected at an angle of 90.degree.. This requires the mirror to be placed at an angle of 45.degree. to the laser beam. Normally, such a mirror is elliptical in cross section with the operative length of the mirror axis at least equal to the beam diameter, and the major axis at least equal to .sqroot.2 times the beam diameter. In this case, the mirror distortion due to thermal bending is a paraboloid of revolution. The mirror bending distortion is proportional to square of the distance from the center of the mirror; and the optical wave front error is likewise proportional to the square of the distance measured from the center of the beam. Since the beam shape on the mirror is elliptical, the distortion at the major axis is greater than at the mirror axis. Since they are not equal, the reflected beam is astigmatic.